This invention relates generally to systems for inspecting printed circuit boards, and more particularly, to an improved system for inspecting printed circuit boards incorporating components with close tolerances.
As is well known to persons skilled in the art, a printed circuit board is used for mounting and electrically interconnecting electrical components in a predetermined manner. To the extent possible, such printed circuit boards are constructed mechanically, using automated assembly machines which operate to reduce the often prohibitive costs of manually assembling a printed circuit board. While reducing overall costs, such automated assembly techniques have been found to give rise to a certain limited number of assembly defects such as incorrect insertions of components, and their leads or pins, as well as defects in the soldering procedures which then follow.
Originally, steps were taken to locate assembly errors of this general type through a visual inspection of each printed circuit board at a desired stage of the manufacturing process, by human operators using the naked eye, or possibly a stereo microscope or the like. However, since this procedure was found to be extremely tedious and inaccurate, as well as a relatively expensive process, steps were taken to develop automated systems for inspecting printed circuit boards, to replace such visual inspections.
Examples of devices of this general type are the Model 5511, Model 5512, Model 5515, Model 5516 and Model 5517 Printed Circuit Board Inspection Systems which are manufactured by Control Automation Incorporated of Princeton, N.J. These inspection devices generally employ a series of cameras which are mounted within a fixture (an inspection head) adapted for controlled movement relative to a printed circuit board. The inspection head is either sequentially advanced to successive viewing fields established along the surface of the printed circuit board then under inspection, or continuously advanced along the surface of the printed circuit board, to acquire images for microprocessor analysis. Any detected defects are in turn reported to the operator, for appropriate correction.
Such devices operate to enhance the accuracy of the inspection process by providing an inspection head which incorporates a series of four angled, orthogonally placed cameras, operated in conjunction with a selectively controllable light source. Through selective control of this series of cameras, and the associated light source, a variety of testing procedures are enabled including a verification of the placement of components (and their leads or pins), both before and after the soldering procedure, as well as a verification of the solder connections which are made.
Initially, such inspections were accomplished by sequentially advancing the inspection head (or the printed circuit board) through successive viewing fields, and by selectively activating the series of cameras and their associated lighting to acquire images for inspection purposes. Later, primarily in order to increase the rate at which circuit board inspections could be accomplished, such inspections were accomplished by continuously advancing the inspection head (or the printed circuit board) through its successive viewing fields, and by selectively strobing the associated lighting system to acquire images for inspection purposes. However, even this enhancement was found to have certain limitations.
Contributing to this was that concurrently with the above-described improvements to the printed circuit board inspection systems, the printed circuit boards to be inspected were themselves undergoing improvement. Techniques were developed for providing a circuit board of a given size with an increasing number of components, for performing a greater number of operations. This gave rise to closer tolerances between the respective components applied to the printed circuit board, closer tolerances between the respective features of a given component (e.g., its leads or pins), and closer tolerances between the soldered interconnections of such components, and their leads or pins, with features of the printed circuit board.
As a result, the various components associated with the printed circuit boards came to be placed closer and closer together (an increased density which is referred to in the industry as an increase in "population"), with a corresponding decrease in tolerance between the various features to be inspected. To accommodate this, it became necessary to magnify the inspections which were being performed. For example, early circuit board inspection systems typically acquired images from successive viewing fields which were on the order of one inch by one inch. Later, these viewing fields were reduced in size, to one-half inch by one-half inch. Further reduction in the size of the viewing fields would, of course, be possible to accommodate a need for higher resolution. However, reducing the size of the viewing fields to be inspected leads to corresponding disadvantages in overall operation.
Most important is that decreasing the size of the viewing fields to be inspected results in a corresponding increase in the overall amount of time which is required to inspect all of the viewing fields defined for a particular printed circuit board. However, also to be considered is that different printed circuit boards will tend to include different component "populations". This can include differences in density for different regions of a particular printed circuit board, or differences in density for different printed circuit boards which are to be inspected by a particular circuit board inspection apparatus. In many cases, this requires the definition of viewing fields of different size, for satisfactory inspections to be performed. Selecting a field of view which is too large can result in an inaccurate inspection, including the failure to identify defects as well as the false identification of defects which do not in fact exist. Selecting a field of view which is too small can unnecessarily compromise the rate at which each printed circuit board is inspected.
Consequently, the optimum field of view for a given inspection can vary widely. This has given rise to the need to change the magnification of the cameras associated with the inspection head of the printed circuit board inspection system, which has in turn limited the ability of available printed circuit board inspection systems to inspect printed circuit boards of varied configuration without requiring significant changes (in hardware and software) from job to job.